Cytoskeleton
Lecture 28
Pages 573 - 607
A cytoskeleton is needed for many many cellular functions,
such as;
Muscle contraction
Permitting sperm to swim
Immune system
In fact without the functioning of the cytoskeleton life would be
stagnant at the very most.
The inside of the cell is also in constant motion, and it is the
cytoskeleton which provides the means to achieve this too.
Mitosis & meiosis
Organelle movements
Cell movement
The cytoskeleton is built on three types of protein filaments:
Actin filaments
Intermediate filaments
Microtubles
Remember: AIM
17_02_3 types of protein filaments
1) Intermediate filaments
Intermediate Filaments
• Have the great tensile strength
• Enable cells to withstand the mechanical stress associated
with stretching
• Called intermediate because their diameter is 10nm
• Toughest and most durable of the three types
• Found in the cytoplasm of most animal cells surrounding the
nucleus and extending out into the cytoplasm
• Found extensively at tight junctions.
• Found within the nucleus too - as part of the nuclear lamina
which supports and strengthens the nuclear envelope
17_03_Interm_filaments.jpg
Intermediate filament structure
• Resemble rope in that many long strands are twisted
together to provide tensile strength
• The strands are made of elongated fibrous proteins
– Each has a N-terminal globular head
– Each has a C-terminal globular tail
– Each has a central elongated rod domain, which is an extended
alpha-helical region. These wrap around each other in a coiled-coil
configuration. These are further associated non-covalently with other
dimers to form tetramers. The tetramers associate together end-toend and side-by-side also non-covalently…
17_04_protein_ropes.jpg
The head & tail units of the fibres bind to other components. The
globular domains vary greatly between different intermediate
fibres
They are very evident in the axons of nerve cells which is a
necessary requirement to prevent its rupture. Also present in
muscle cells and epithelial cells. Shown below are the potential
consequences of cells lacking these structures
17_05_strengthen_cells.jpg
The most diverse subunit family are the keratins, with different
sets present 17_06_filam_categories.jpg
in different epithelia e.g. the lining of the gut as
opposed to the skin, hair, feather, claws, desmosomes
The intermediate filaments are stabilized by accessory proteins
17_07_Plectin.jpg
that transverse them and links them to other cellular structures.
One important type is plectin.
Application: Nuclear enveolpe
The nuclear membrane come and goes as
the cell performs the cell cycle.
The nuclear envelope is supported by a meshwork of
intermediate filaments which are formed from lamins (not to
be confused with laminin that is the extracellular matrix
protein.
17_08_nuclear envelope.jpg
Interestingly, the assembly and disassembly of the lamina is
controlled by dephosphorylation and phosphorylation,
respectively, of the lamins by protein kinases, i.e. during
mitosis
2) Microtubles
Microtubles
• Microtubles have a crucial role in all eucaryotic cells
• Long stiff hollow tubes of protein that disassemble and
reassemble to various locations in the cell.
• In most cells they tend to grow from a central location within
the cell called the centrosome. [it should be very clear now
that the spelling of names in biology is very important centriole, centromere, centrosome, etc.]
• They extend out towards the cell periphery
• They create a system of tracks across the cell
• Vesicles, organelles, and other cell components travel like
trains along these tracks
• Keeping things in their places too, such as organelles
The microtubles
reform as the mitotic
spindle during cell
division along which
the chromosomes
move.
17_09_Microtubules.jpg
They also form the
core of the cilia and
flagella that beat
rhythmically.
Microtubule structure
• Microtubules are built from subunits - tubulin
• Tubulin is made of two subunits - alpha-tubulin and betatubulin bound together by ionic bonds.
• These subunits stack together into long fibers microtubles
• These fibers from a cylinder made of 13 parallel microtubles
- each is called a protofilament
• These protofilaments have a polarity
– at one end there are just free alpha-tubulins - this is known as the
minus end
– at the other beta-tubulins - this is knowna s the plus end
– this directionality is akin to defining one way streets along which
material is transported…
17_10_tubes_tubulin.jpg
Centrosome
• Microtubules are formed by outgrowth from the specialized
organizing center which controls
– The number of microtubules formed
– The location of these mictotubules
– The orientation of these microtubules
• It is known as the centrosome
• It is typical present on one side of the nucleus
• It is a complex structure composed of gamma-tubulin
subunits
• It does contain the centrioles (which are not involved in
microtubule formation
• Alpha/Beta subunits grow from it…
17_11_centrosome.jpg
The microtubules
grow and contract in a dynamic fashion.
17_12_grows_shrinks.jpg
This is known as dynamic instability and use a great deal of
ATP
Microtubules carry cargo along their lengths, e.g. axons of nerves
It is estimated that they move material at a rate of 10 cm per day
17_15_nerve_cell_axon.jpg
Motor Proteins
• Microtubules have a lot of associated proteins
• One class are the motor proteins which bind to both actin
and intermediate filaments
• They use the energy of ATP hydrolysis to travel along the
filaments (for both actin and intermediate)
• They attach with their globular end to the filament
• They attach their other end to the cargo they carry
• Dozens have been identified
• THEY DIFFER IN THE CARGO THEY BIND AND THE
DIRECTION THEY TRAVEL
• Kinesins move towards the plus end
• Dyneins move towards the minus end…
17_17globular heads.jpg
17_18_motor_proteins.jpg
Cilia and flagella contain stable microtubules moved by dynein.
17_24_cilia.jpg
Cilia are small outgrowths
on the cell surface that beat to move
fluid across the cell - mucus in the lungs
Flagella are longer structures that propel the entire cell.
The microtubules of these two structures are
slightly different in that they have a ‘9 + 2’
arrangement.
17_28_dynein_flagell.jpg
3) Actin Filaments
Actin Filaments
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•
•
•
•
•
•
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Found in all eukaryotic cells
Essential for movement - cell crawling, dividing into two.
Unstable dynamic structures
Also able to bind with a whole bunch of alternative actin-binding proteins
to allow them to serve a variety of functions
Act like the internal muscles of the cell - pulling the cell into various
shapes
They are thin and flexible - about 7 nm in diameter
Each is a twisted chain of identical globular actin molecules - all pointing
in the same direction - so they have a plus and minus side also.
Much longer than microfilaments in total length as there are many more
subunits of actin in the cell than microfilament subunits (x 30 times)
17_29_Actin_filaments.jpg
17_30_protein threads.jpg
17_33_move_forward.jpg
All actin-dependent motor proteins
belong to the myosin family - which
use ATP hydrolysis to generate
movement along the filaments
17_38_myosin_I.jpg
Muscle belongs to the myosin-II subfamily of myosin - these
have two ATPase heads. Clusters of these bind together to
make the myosin filament
17_40_Myosin_II.jpg
17_41_slide_actin.jpg
17_44_Muscles contract.jpg